A common problem with sail boats provided with an auxiliary engine is the movement of the propeller and shaft while the sail boat is under sail. This effect, known as windmilling, creates a number of problems. The movement of the propeller and shaft creates unnecessary wear and damage to the transmission, coupling and shaft bearings. The rotating shaft creates a loud and irritating noise to those on board. Also the speed of the vessel while under sail is decreased anywhere from 5 to 15 percent when the shaft is allowed to rotate rapidly and creates drag. By locking the shaft and positioning the propeller behind the keel or "dead wood", the drag is effectively eliminated. Prior propeller shaft locking devices as in U.S. Pat. Nos. 3,831,547 and 3,786,775 are either complicated devices, difficult to install, or both.